1. Field of the Invention
The present invention relates to a system for estimating traffic information of a road link where data is not collected by a probe car.
2. Description of Relevant Art
A probe car system can collect wider traffic information with a lower cost compared with, such as, VICS (Vehicle Information and Communication System) which collects the traffic information by on-road sensors. However, since a running position and running timing of the probe car are probabilistic, space and time missings occur in a data series of collected probe traffic information. For example, if we focus on time-series data of the traffic information in one road link, since the probe car may be running in some case and may be not in other case depending on a time, the time-series data of the collected traffic information frequently contains a missing value. In addition, if we focus on a plurality of road links at a certain moment, since the probe car may be running in some road link (road link where the traffic information is collected) and may be not in other link (road link where the traffic information is not collected), a spatial data series also contains a missing value. For example, in the application for providing with information to a car navigation system or a path search, if there is a missing in the traffic information, correct processing of the information is difficult. Therefore, it is requested to provide with some estimated information to the link where the traffic information is missing if the traffic information is used for the above application.
A method for estimating the traffic information of another road link from that of collected by on-road sensors, such as VICS, is disclosed, for example, in Japanese Laid-Open Patent Application Number 7-129893. This method estimates the traffic information of a link where the traffic information is missing from upstream and downstream links, or from the traffic information of a link which is parallel to the link, based on a connection relation of the road link. On the other hand, a statistical usage of the probe traffic information is described in a non-patent document, “A NEW INFORMATION PROVIDING SYSTEM EXPANDING POSSIBILITY OF CAR NAVIGATION” (Tsuge, et al.), “JIDOSHA GIJUTSU” (Car Technologies), Vol. 58, No. 2, pp44-48, 2004/2, as an estimation method which uses only the probe traffic information, without depending on the connection relation of the road link. This method stores the probe traffic information after processing it into traffic information in conformity with VICS regulations, and provides with current traffic information when the current traffic information is collected, or past traffic information, which has been statistically processed, instead of the current traffic information when the current traffic information is not collected. Other than the above, for example, there is a method for continuing providing past probe traffic information until the probe traffic information is updated as a simple estimation method.
However, there are following problems in these conventional estimation technologies. For one thing, when a percentage of missing values (missing percentage) occupying within a data series of the probe traffic information is high, an estimation based on the connection relation of the traffic link is difficult. The missing percentage, when it is a missing percentage of time, is a ratio of a number of times which could not collect the probe traffic information during an update period to a number of update times of the probe traffic information per day for a road link. Also, a spatial missing percentage is a ratio of a number of road links which could not collect the probe traffic information during the update period of the probe traffic information to a number of total road links included in a control unit (for example, a unit of map mesh) of the probe traffic information. For example, even if one hundred thousand probe cars are prepared throughout Japan, a number of update frequencies of the probe traffic information will be one time per hour in average for one road link. If we try to use the probe traffic information at every five minutes as the traffic information, which is almost the same condition with the VICS, the spatial missing percentage will be over 90%. Therefore, when an estimation of the traffic information of some road link is intended by using neighbor road links, it frequently happens that the traffic information of the neighbor road links is entirely missing. In addition, if the estimation is implemented based on the connection relation with distant road links, the estimation accuracy is rapidly decreased, thereby resulting in large discrepancy between the estimated information and a current traffic status. On the other hand, if the past probe traffic information is utilized statistically, the estimation is possible even if the missing percentage of the probe traffic information is high. However, the probe traffic information, which has been statistically processed, does not always indicate the current status.
It is, therefore, an object of the present invention to provide a traffic information system which accurately reflects current probe traffic information, which is collected from another road link, in an estimation of traffic information of a road link where the current probe traffic information is not collected, when the probe traffic information with high missing percentage is utilized.